1. Field of the Invention
The invention relates to voltage measurement of high voltage batteries for hybrid and electric vehicles.
2. Background Art
Hybrid and electric vehicles use high voltage batteries to supply power to electric motors and other electric components inside the car. Vehicles also still include low voltage batteries (for example, a 14 volt battery). The control of these electric modules is on the low voltage side of the vehicle electrical system. There is a requirement to have isolation between the high voltage battery side and the low voltage battery side of the system.
The high voltage batteries are referred to as traction batteries as they provide energy for propulsion of the vehicle. In a hybrid or electric vehicle, accurate monitoring of the individual voltages of the traction batteries is very important.
Accurate monitoring facilitates effective charging of the batteries, as well as effective control over the discharging of the batteries as the vehicle operates. An important feature of a battery monitoring system is to maintain the voltage isolation typically provided between the traction batteries and the low voltage portion of the vehicle electrical system, which is typically referred to as chassis ground.
There are existing approaches to measuring voltage of traction batteries, for example, with voltage divider networks or switched inputs to a differential amplifier, and maintaining voltage isolation. There are already circuits designed to sense battery voltage with an isolation barrier, but most of these circuits use analog signals, and therefore they have to provide an isolation between battery side and vehicle side (chassis). The existing circuits use analog components such as linear analog optocouplers or isolated analog amplifiers.
A problem with the kind of components used in these existing circuits is that in some cases several problems of nonlinearity and variations of transfer gain during the life of the component can occur causing measurement error. In these existing circuits, where high accuracy is required, the use of expensive components is expected.
The problem of passing analog signals across an optocoupler has been recognized. An approach to measuring battery voltage has been contemplated in which an amplifier output is coupled to an analog-to-digital converter, which in turn, is coupled to the optocoupler.
Background information may be found in U.S. Pat. Nos. 5,712,568 and 6,362,627.